1. Field of the Invention
The present invention relates to a mobile track tamping machine for simultaneously tamping ballast under a plurality of track ties defining a distance between them and fastened to rails of the track, which comprises a machine frame extending in a longitudinal direction and supported by undercarriages on the track for movement in an operating direction, two tamping tool assemblies sequentially arranged in the longitudinal direction, each tamping tool assembly being capable of simultaneously tamping ballast under at least two track ties and comprising two tamping tools adjacent each other in the longitudinal direction and reciprocable in this direction from an initial position to a tamping position, the two tamping tools defining the distance between the track ties in the initial position, and a reciprocating drive connected to the two tamping tools. The machine further comprises a track lifting and lining unit, and a reference system controlling a correction of the position of the track. This invention further relates to a machine arrangement incorporating such a machine and to a tamping method.
2. Description of the Prior Art
A two-machine arrangement for track tamping in switches has been disclosed in U.S. Pat. No. 5,379,700. It comprises a first machine with a tamping tool assembly capable of simultaneously tamping ballast under a plurality of ties, and an auxiliary machine which may be coupled to the first machine and carries a tamping tool assembly capable of tamping a single tie. Before the tamping operation in a switch is started, the self-propelled auxiliary machine is uncoupled. Those ties in the main track and the branch track where the tamping tools may be readily immersed in the ballast are then tamped with the first machine while the ties that could not be readily tamped with the first machine are tamped with the auxiliary machine without lifting of the track. This permits efficient tamping of switches.
U.S. Pat. No. 3,744,428 discloses a two-machine arrangement wherein each track tamping machine carries a tamping tool assembly capable of simultaneously tamping ballast under two sequentially arranged ties. The rearward tamping tool assembly is displaceable in the longitudinal direction. In a first operating stage, two adjacent ties are simultaneously tamped by the leading tamping tool assembly, and the entire machine arrangement is then advanced by four ties by skipping two ties. The two skipped ties are then tamped by the rear tamping tool assembly. In an embodiment shown in FIG. 7 of the patent, both track tamping machines carry tamping tool assemblies capable of tamping only a single tie, and these assemblies are spaced a distance corresponding to the distance between two ties, i.e. a crib width. To compensate for differences in crib widths, the tamping tool assemblies are mounted for displacement in a longitudinal direction.
Track tamping machines with two tamping tool assemblies capable of simultaneously tamping more than one tie are also disclosed in U.S. Pat. Nos. 3,494,297, 3,595,170 and 4,224,874.